1. Field of the Invention
The present invention relates to devices useful in determining the activity of an ionic analyte of an aqueous solution, and is particularly useful in potentiometrically measuring ion activity in drops of biological fluids.
2. Description of the Prior Art
There is a variety of apparatus in the prior art for measuring ion activity in solutions. A test device incorporating ion-selective electrodes which develop an electrical potential proportional to the logarithm of the activity of the ions to which the electrodes are sensitive is described in the abovementioned commonly assigned U.S. patent application Ser. No. 927,085 and is shown at 10 in FIG. 1 of the accompanying drawings. Two solid electrodes 12 and 14 are mounted on a frame 16, and a capillary bridge 18 is provided for promoting ionic migration between two fluid access holes 20 and 22 at the electrodes. The capillary bridge includes a nonporous support layer, a porous layer with ionic access to both electrodes, and a top nonporous cover layer which is preferably hydrophobic. When a drop of reference solution of known ion activity is applied to one fluid access hole and a drop of test solution is applied to the other fluid access hole, the drops spread into the porous layer until contact is made at a thin junction interface, permitting ionic migration between the drops. An electrometer 24 is provided to measure the electrical potentials at the interfaces between each solution drop and its associated electrode to provide an indication of ion activity in the test solution.
Although the device disclosed in U.S. patent application Ser. No. 927,085 provides excellent results in determining ion activity in liquids, the present invention is an improvement which provides advantages in both assembly and performance. As can be seen in FIG. 1, capillary bridge 18 is a small discrete part which must be both accurately placed on the electrodes during assembly and held in place by an adhesive. These constraints increase assembly problems and potentially decrease performance due to possible misalignment of the fluid access holes and failure of the adhesive bond. Further, fluid leakage from the edge of the bridge could affect the area of wetted electrode surface because the edges of the bridge are aligned with the electrodes. Unpredictability of the total area of wetted electrode surface would adversely affect the test results.